Navigation system for and method of tracking the position of a work target

ABSTRACT

A navigation system for and method of tracking the position of the work target is provided. The navigation system can detect distortions of a trackable device during the navigation procedure and compensate for such deformation in a way that reduces or eliminates navigational error and/or avoids or reduces the need to re-set the navigation system during a navigation procedure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European Patent Application 14 001698.1, filed May 14, 2014, and entitled “Navigation System for andMethod of Tracking the Position of a Work Target”, the entirety of whichis incorporated herein by reference.

TECHNICAL FIELD

The present disclosure is directed toward a navigation system fortracking the position of a work target with a trackable device, such asa surgical navigation system. The disclosure is also directed toward amethod of tracking the position of the work target using such anavigation system.

BACKGROUND

Navigation systems for tracking the position of one or more work targetslocated inside a body, either alone or in relation to one or morevarious working tools, are used in many types of applications. Oneapplication in which navigation systems are commonly used is in thefield of navigated surgical procedures. In this field, surgicalnavigation systems are now commonly used to assist in planning asurgical procedure or executing a planned surgical procedure, so as toimprove the accuracy of the surgical procedure and minimize theinvasiveness thereof.

In some surgical navigation systems, a rigid trackable device is securedto the patient so as to be fixed in relation to some portion of thepatient's body. A computer implemented tracking system is used to trackthe position of the trackable device and, based on a previouslydetermined spatial relationship between the trackable device and somearea of interest within the patient, thereby track the position of awork target within the patient. The computer-implemented tracking systemis also used to track the position of a surgical working tool, forexample by means of a second trackable device secured to the workingtool. From these two tracked positions, the navigation system thereby isable to track the position of the working tool relative to the workingtarget within the patient. The tracking information is then correlatedwith previously defined surgical plan information relative to one ormore intra- or pre-operative scan information data sets, such as CT(computer tomography) images, MRI (magnetic resonance imaging) images,X-ray images, ultrasound image data, or other similar information aboutthe body of the patient, by one or more methods of registering thepre-operative scan information data to the intra-operative trackinginformation.

A limiting design parameter of such a navigation system is that thetrackable device usually must be invasively attached to the patient,such as with pins or other fasteners securely fastened to a bone. Suchsecurement can lead to additional possibilities for potential problemswith post-surgical recovery of the patient. Additionally, the variousnavigational routines implemented by the computer-implemented trackingsystem are based on the assumption that the trackable device does notmove relative to the patient and the target area during a navigationprocedure. Therefore, movement of the trackable device relative to thepatient during a navigation process, such as by accidental bumping, lossof fixation, or other means, can lead to excessive errors and require acomplete re-setting or reconfiguration of the navigation system, whichcan use up valuable time during a surgical procedure.

Other surgical navigation systems use a trackable device formed of aflexible substrate with several LEDs (Light Emitting Diodes) carried bythe flexible substrate. The flexible substrate is secured to the skin ofthe patient in a manner and location to prevent or minimize deformationor movement relative to the features of the patient. Thus, the trackabledevice is generally attached to very bony areas of the patient, such asthe skull and/or nose portions so as to remain in a fixed positionrelative to a work target inside the patient once a navigation procedurehas begun. A limitation with such a navigation system, however, is thatany deformation of the trackable device once a navigation procedure hasbeen started can lead to excessive navigation errors and or requirereconfiguration of the navigation system.

SUMMARY

According to some aspects, a navigation system for and method oftracking the position of the work target is provided, wherein thenavigation system can detect distortions of a trackable device duringthe navigation procedure. In some circumstances, the navigation systemcan compensate for such distortions in a way that reduces or eliminatesnavigational error and/or avoids or reduces the need to re-set thenavigation system during the navigation procedure.

According to some aspects, a navigation system for tracking the positionof a work target located inside a body that is compressible and has adistortable outer surface is provided. The navigation system includes atrackable device and a computer-implemented tracking system. Thetrackable device includes a plurality of tracking points configured tobe secured to the outer surface of the body. The tracking points areconfigured to be moveable relative to each other when secured to theouter surface of the body. The computer-implemented tracking system isadapted to remotely track the positions of each tracking point relativeto a coordinate system. The tracking system includes a computerprocessor arrangement adapted to implement a navigation routine thatincludes the steps: accessing an initial model of the trackable device,the initial model having an initial shape based on initial locations ofa set of the tracking points; registering the initial model with aninitial position of the work target in an image of the work target;sensing a deformation of the trackable device with the tracking systemafter registering the initial model; creating a refined model of thetrackable device that compensates for the deformation; and calculatingthe current position of the work target from the refined model.

According to some aspects, a method of tracking the position of a worktarget located inside a body with a navigation system includes thesteps: accessing an initial model of the trackable device, the initialmodel having an initial shape based on initial locations of a set of thetracking points; registering the initial model with an initial positionof the work target in an image of the work target; sensing a deformationof the trackable device with the tracking system after registering theinitial model; creating a refined model of the trackable device thatcompensates for the deformations; and calculating the current positionof the work target from the refined model. The method may be implementedby a computer-implemented tracking system, such as with hardware and/orsoftware, configured to remotely track the positions of tracking pointsof a trackable device relative to a coordinate system. The method may beimplemented with the use of a trackable device having a plurality oftracking points configured to be secured to the outer surface of thebody, wherein the tracking points are configured to be moveable relativeto each other when secured to the outer surface of the body.

These and other aspects may include any one or more of the variousoptional arrangements and features described herein.

In some arrangements, the steps of accessing and registering may beperformed only one time during a navigation procedure, such as at ornear the beginning of the procedure. The steps of sensing a deformation,creating a refined model, and calculating the position of the worktarget may be performed one or more times, such as iteratively, duringthe course of a navigation procedure.

In some arrangements, the step of creating a refined model may includeremoving one or more of the sensed tracking points of the set and/oradjusting a sensed location of a tracking point relative to sensedlocations of the other tracking points in the set. For example, the stepof creating a refined model may include removing at least one of thetracking points of the set that is deformed more than a deviationthreshold from the initial shape or any other reference. Also by way ofexample, the step of creating a refined model may include adjusting asensed location of a tracking point relative to sensed locations of theother tracking points in the set as a function of a spatial deviationbetween the sensed location of the tracking point and the initiallocation of the tracking point and/or optionally relative to the initiallocations of one or more of the other tracking points.

In some arrangements, sensing the deformation may include trackingsubsequent locations of the tracking points of the set after the initialmodel is registered, the subsequent locations defining a deformed shape.A deformed model of the trackable device may be based on the sensedlocations of the tracking points and the deformed shape. Then thedeformation of the trackable device may be identified based on adifference between the deformed shape and the initial shape.

In some arrangements, the step of identifying the deformation includesmatching the initial shape to the deformed shape and calculating aspatial deviation of a tracking point of the set. Optionally thematching may be performed by transforming the initial locations of thetracking points to the subsequent locations of the tracking points andcomparing the same points in the two models to each other to identifythe spatial deviation. For example, initial coordinates of the trackingpoints (e.g., the initial shape) may be transformed to attempt to matchmeasured coordinates in a point cloud of sensed tracking points (e.g.,the deformed shape). This transformation may be an orthogonaltransformation. Such a transformation and matching may include a leastsquares matching algorithm to find a best-fit comparison. The matchedshapes may then be compared to calculate spatial deviations of thepoints, such as the absolute value of the distance between thecoordinates of one or more of the measured tracking points and thetransformed coordinates of the initial locations of the same trackingpoint. Alternatively or additionally, the coordinates of the sensedtracking points may be transformed and matched to the coordinates of theinitial locations of the same tracking points, or other coordinatetransformations suitable for matching and comparing the initial anddeformed shape may be used.

In some arrangements, the step of creating a refined model includesexcluding the sensed tracking point from the set when the spatialdeviation for that tracking point exceeds a defined spatial deviationthreshold and/or has a spatial deviation more than at least one othersensed tracking point in the set. Optionally, the tracking point isexcluded only when its spatial deviation exceeds the defined spatialdeviation threshold and has the largest spatial deviation of all thetracking points in the set, whereby only one (e.g., the most deformed)tracking point is excluded at a time. The sensed tracking point may beexcluded by removing it from the set of tracking points used to form therefined model. Thus, the refined model may be based on a reduced set ofthe sensed tracking points (relative to the initial model and/or thedeformed model) without the subsequent location of the removed trackingpoint. Optionally, the deformed tracking point may be removed from theset of sensed tracking points without modifying the shape of remainingportions of the deformed model.

In some arrangements, the step of creating a refined model includesadjusting the position of a sensed tracking point when it is identifiedas being deformed. The location of the deformed tracking point in thedeformed model may be adjusted into a corrected position in the refinedmodel. The corrected position of the deformed tracking point may thus beretained in the refined model rather than omitting the deformed trackingfrom refined model. The sensed location of a tracking point may beadjusted relative to the sensed locations of the other tracking pointsin the set as a function of the relative positions of the trackingpoints to each other and the spatial deviations of these relativepositions and/or relative distances. The adjusting may be performedbased on a force spring model assumption of movements of the skin of apatient in a finite element method (FEM) analysis. The spatial deviationof a deformed tracking point may be equated to a resultant force. A setof forces applied to the force spring model may be calculated to shiftthe deformed tracking point toward the original position in the originalmodel. The resultant shifting of one or more of the remaining trackingpoints may also be modeled. One or more of the sensed tracking pointsmay be held to maintain a fixed position relative to the work target,while other ones of the sensed tracking points are shifted based on theforce spring model in the FEM analysis.

The deviation threshold may be a predefined static value and/or be adynamic value dependent on selectable factors. Optionally, the deviationthreshold can be based on a comparison of the spatial deviation for thetracking point relative to an averaged spatial deviation for up to allof the tracking points in the set. An averaged spatial deviation mayinclude a direct average, a weighted average, and/or another type ofaverage. A tracking point may be called and/or considered a deformedtracking point and/or considered to have an unacceptable location errorwhen its spatial deviation exceeds such a deviation threshold and/orexceeds the spatial deviation of other ones of the tracking points, forexample, by having the largest spatial deviation of all the trackingpoints. The decision of whether to remove and/or adjust the sensedlocation of a tracking point in the refined model may depend uponwhether the tracking point is considered to be a deformed tracking pointand/or have an unacceptable location error.

In some arrangements, before the current position of the work target iscalculated, the steps of matching the initial shape to the deformedshape, calculating a spatial deviation, and removing a tracking pointare iteratively repeated. In subsequent iterations, the matching isperformed with a reduced number of tracking points due to the exclusionof the previously removed tracking point. These steps may be iterativelyrepeated until at least no further sensed tracking points are removedfrom the set of tracking points and/or the set of sensed tracking pointsincludes fewer than a pre-defined minimum number of tracking points. Thepre-defined minimum number of tracking points may be a staticallydefined number or may be a dynamically defined number, for example basedon a function of parameters obtained during the navigation procedure.Optionally, the navigation routine provides a notification to a user ifand when the set of tracking points included in the refined modelincludes fewer than the pre-defined minimum number of tracking points.Such a notification may include providing an audible or visiblenotification, such a warning, and/or automatically terminating orsuspending the navigation routine.

The navigation routine may be adapted to provide a notification to auser in when one or more error thresholds are met or exceeded. In somearrangements, the navigation routine estimates an expected error of thecurrent position of the work target calculated from the initial modeland the deformation of the trackable device and/or the reduction oftracking points in the refined model. In some arrangements, thenavigation routine calculates an averaged spatial deviation of some orall the tracking points in the set of tracking points from the spatialdeviations. Optionally, the navigation routine may provide an indicationto a user when the expected error exceeds a pre-defined maximum errorthreshold for the work target and/or the averaged spatial deviationexceeds a pre-defined maximum error threshold. The pre-defined errorthresholds may be statically defined numbers and/or may be a dynamicallydefined numbers, for example based on a function of parameters obtainedduring the navigation procedure. The indication may include a warningmessage and/or ending the navigation routine, such as by terminating orsuspending the navigation routine. The warning message may include avisual message, such as with a visual warning provided on a displayscreen, an audible message, such as with an audible alert provided via aspeaker, and/or another type of message that is designed to attract theattention of the user.

In some arrangements, the navigation routine senses the initiallocations of the tracking points of the set of tracking points and/ormay import the initial locations from some other source, such as amemory or database. The sensing may be performed with a navigationsensor configured to measure a position of the tracking points relativeto the navigation sensor. Such a navigation sensor may, for example,include one or more cameras and/or a magnetic position sensor, althoughother types of electronic navigation sensors could also or alternativelybe used. The sensing may be performed by detecting with and/orextraction from a pre- or intra-operative scan image, such as a CT scan,MRI, X-ray, ultrasound, optical, or other similar imaging modality. Thenavigation routine may create the initial model of the trackable devicefrom the initial locations of the set of tracking points, or the modelmay be imported from some other source, such as a memory or database.

Registration of the initial model of the trackable device with aninitial position of the work target in an image of the work target maybe performed in any suitable manner. In some arrangements, the step ofregistration includes identifying an initial pose (i.e., a position andorientation of a three-dimensional shape relative to a three-dimensionalcoordinate system) of the initial model relative to a global coordinatesystem from the initial locations of the set of tracking points, andregistering the initial pose of the initial model to the initialposition of the work target in an image of the work target, such as anMRI, CT, X-ray, ultrasound, optical, or other similar imaging modality,relative to the global coordinate system. Suitable registration methodsthat may be used, include surface matching techniques and point-to-pointmatching techniques. The navigation routine may include an automaticregistration feature, such as a shape matching routine or any knownautomatic registration routine, and/or may include a manual registrationfeature, such as by a point-to-point registration routine or any knownmanual registration routine. However, any suitable registrationprocedure may be used.

The trackable device may include a flexible substrate configured to besecured to the outer surface of the body, however, a rigid trackabledevice could also be used. One or more of the tracking points may becarried by the same flexible substrate or by separate flexiblesubstrates. The flexible substrate may have almost any shape, such as asheet having a relatively thin thickness with opposite first and secondsides and one or more peripheral edges. The flexible substrate may havecomplete flexibility in all directions, limited flexibility in only somedirections, and/or have regions of complete flexibility and regions oflimited flexibility. In some arrangements, the flexible substrate may bein the shape of a frame surrounding a window through a central portionof the trackable device. The frame may have the shape of any closed orsemi-closed shape, such as a rectangle, a square, a circle, an oval, aU, a semi-circle, or an H. The window may be sized and shaped to allowthe frame to partially or completely surround a surgical area on thepatient's skin over the work target. The window may be sized and shapedto provide a sufficient space through which a surgical procedure on thework target can be performed. Optionally, the number of tracking pointson the flexible substrate may be between twenty and forty opticalemitters disposed on the flexible substrate and extending around thewindow. However, more or fewer tracking points may also be used. In somearrangements, the trackable device includes two or more flexiblesubstrates configured to be secured separately to the outer surface ofthe body, for example by being spaced apart from each other. Each of thesubstrates may carry only one, or may carry more than one trackingpoint, for example between 2 and 20. Each flexible substrate may be inthe form of a patch and may have almost any shape, such as rectangular,circular, oval, and so forth. The flexible substrates may have othershapes, such as an elongate strip, cross, or star. In this manner, anarray of separate flexible substrates may be located individually on theskin of a patient in the area around a work target in almost any shapeor configuration desired. The tracking point or points may be disposedon one side of the flexible substrate. An adhesive may be disposed onthe other side of the flexible substrate to allow the substrate to besecured to the outer surface of the body. The adhesive may be abio-compatible adhesive suitable for securing the substrate to the skinof a patient without injuring the patient and allowing the substrate tobe subsequently safely removed from the skin. The trackable device maybe applied to the surface of the body, such as the skin of the patient,such that the centroid of the sensed of tracking points, that is, thecentroid of the sensed point cloud, is very close to the working targetthat is to be tracked.

The tracking points may be any feature or structure adapted to be sensedby the computer-implemented tracking system. In some arrangements, thetracking points include an LED, a reflective surface, a reflectivepattern, a magnetic coil, and/or an optically identifiable geometricshape that uniquely defines position and orientation.

A work piece may be adapted to be tracked by the tracking system,wherein the tracking system is adapted to track the position of the workpiece relative to the coordinate system. The navigation routine furtherinclude the step of calculating the position of the work piece relativeto the position of the work target based on the tracked position of thework piece and the calculated position of the work target.

In some arrangements, the trackable device includes a data communicationlink suitable for sending and/or receiving data to and from thecomputer-implemented tracking system. The optical emitters may beselectively activated in response to command signals received from thetracking system through the data communication link. Informationregarding physical constraints of how the tracking points can moverelative to each other, including at least one of flexibility of thesubstrate, rigidity of the substrate, and type of connection betweentracking points may be associated with the trackable device. Theinformation regarding physical constraints may be communicated to thecomputer-implemented tracking system by the data communication link,and/or the information may already be stored on the computer system.

In some arrangements, the navigation system includes a computerprocessor in data communication with one or more tracking sensors. Thedata connection may be wired and/or wireless. The navigation routine maybe implemented by hardware and/or software accessed by or installed onthe computer processor directly or run remotely, such as via an internetconnection to one or more other computer processors. The navigationroutine may be stored in one or more non-transient computer memory, suchas RAM or ROM. Additional data, such as scan image data, procedureplanning data, user preferences, patient data, and other useful data maybe stored, for example, in an electronic database also provided innon-transient computer memory. The computer processor may have access tothe navigation routine and/or the database so as to be able to implementthe methods and processes described herein. Input/output devices, suchas a keyboard, mouse, display monitor, may be operatively connected withthe computer processor to facilitate ease of use by a user. Thenavigation system may have a data connection that allows for remoteaccess and/or control, for example via an internet, local area network,wide area network, or similar data connection. Other common computerhardware and/or software helpful, necessary, and/or suitable forimplementing computerized, planning, control, and/or execution of anavigation procedure may be included as part of the navigation system ina manner well understood in the art.

In some arrangements, the navigation system is a surgical navigationsystem adapted for use in a surgical operating theater to help navigatea surgeon through a surgical procedure. The trackable device may beadapted to be attached to the skin of a surgical patient and to extendaround a surgical area on the patient without covering the surgicalarea. The surgical area may include one or more of a bone and an organ,such as the spine, a lung, the liver, a femur, and the pelvis. However,the work target may include other bones, organs, and/or items disposedin, on, or near the body.

The computer-implemented tracking system may include sensors for sensingthe tracking points in the form or one or more cameras. In somearrangements, the computer-implemented tracking system may include aplurality of cameras. Each camera may be adapted to capture a view ofthe tracking points, and the computer processor arrangement may beadapted to calculate the location of each tracking point relative to thecoordinate system by triangulation of the views of the tracking points.The cameras may be supported independently of the body, a working tool,and/or a user. In some arrangements, the cameras are carried by a framethat is adapted to be secured to, for example, a wall, a supporttrolley, or other structure. In some arrangements, one or more camerasmay be carried by a surgical tool, and the computer processorarrangement may be adapted to calculate a pose of the tool relative tothe work target based on images of the trackable device captured by theone or more cameras. In some arrangements, one or more of the trackingpoints may include an optical target that uniquely defines a pose. Thecomputer-implemented tracking system may include a camera adapted tocapture an image of the optical target, and the computer processorarrangement may be adapted to implement a tracking routine thatcalculates the pose of the optical target from the captured image.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects and features of the navigation system andmethod presented herein may be understood further with reference to thefollowing description and drawings of exemplary arrangements.

FIG. 1 is a schematic diagram of a navigation system according to anexemplary arrangement of the present disclosure;

FIG. 2 is a logic flow diagram of a navigation routine implemented bythe navigation system of FIG. 1 according to an exemplary method;

FIG. 3 is a schematic representation of an initial model of an exemplarytrackable device in in the navigation system of FIG. 1;

FIG. 4 is a schematic representation of registering the initial model ofFIG. 3 with image data of the work target;

FIG. 5 is a schematic representation of sensing deformations of thetrackable device in FIG. 1;

FIG. 6 is a schematic representation of a refined model of the trackabledevice in FIG. 1;

FIG. 7 is a logic flow diagram depicting some optional arrangements ofthe navigation routine of FIG. 2.

FIG. 8 is an isometric view of the exemplary trackable device shown inFIG. 1;

FIG. 9 is an isometric view of another exemplary trackable device usablein the navigation system of FIG. 1 according to additional aspects ofthe present disclosure;

FIG. 10 is an isometric view of a further exemplary trackable deviceusable in the navigation system of FIG. 1 according to aspects of thepresent disclosure; and

FIG. 11 is an isometric view of still a further exemplary trackabledevice usable in the navigation system of FIG. 1 according to aspects ofthe present disclosure;

DETAILED DESCRIPTION

Turning now to FIGS. 1 and 2, a navigation system 20 is arranged fortracking the position of a work target 22 located near a body 24. Thework target 22 may be located inside the body 24, on the outer surfaceof the body 24, and/or even spaced from the body 24. The navigationsystem 20 includes a trackable device 26 that is adapted to be securedon the exterior of the body 24, and a computer implemented trackingsystem 28 which is adapted to track the position of the trackable device26 during a navigation procedure and to detect and compensate for somedeformations of the trackable device 26 during a navigation procedure.The navigation system 20 is particularly well suited for tracking theposition of the work target 22 wherein the body 24 is compressibleand/or has a distortable outer surface. However, the navigation system20 is suitable for use with a body 24 that has any shape and figuration,including having a fixed shape. In the depicted example, the navigationsystem 20 is a surgical navigation system adapted and arranged fortracking the position of a work target inside of a patient's body, andpreferably relative to one or more working tools, such as at 40, and/oroperative plans. In the depicted example, the navigation system 20 isparticularly adapted for surgical navigation on a human. However, thenavigation system 20 may be used for surgery on other animals, and/ormay be adapted for tracking the position of the work target in othertypes of bodies that are not humans or animals. The surgical navigationsystem 20 is adapted to track the position of a bone and/or an organdisposed inside of the patient 24. Thus, the work target 22 may includebones of a spine, a lung, a liver, a femur, a pelvis, an orthopedicimplant, or any other object disposed inside the body. However, thenavigation system 20 may also be used to track the position of worktargets disposed outside of the body, such as on the skin of the patient24. In the description of this particular exemplary arrangement, thework target 22 may be a vertebrae or a portion of the patient's spine,it being understood that other work targets are also contemplated.

The trackable device 26 includes a plurality of tracking points 30 thatare configured to be secured to the outer surface of the body 24, suchas the outer surface of the skin of a patient. The tracking points 30are moveable relative to each other, such as by deforming relative toeach other, when secured to the skin of the patient. The trackabledevice 26 is not limited to a particular shape or form, and may berigid, flexible, and/or have multiple separate sections. Some exemplaryshapes and forms of the trackable device 26 suitable for use in thenavigation system 20 are discussed in detail hereinafter with referenceto the trackable devices 26 a-d shown in FIGS. 8 to 11. For purposes ofexample only, the trackable device 26 in FIG. 1 has a plurality oftracking points 30, such as LED's, disposed on a flexible substratehaving the shape of a generally rectangular frame with an open windowthere through that can be removably secured to the patient's skin withadhesive, generally similar to the trackable device 26 a shown anddescribed hereinafter with respect to FIG. 8. However, the trackingdevice 26 is not limited to this particular arrangement.

The computer-implemented tracking system 28 is adapted to remotely trackthe position of one or more of the tracking points 30 relative to acoordinate system 32. The coordinate system 32 may be any coordinatesystem suitable for use with the tracking system 28, such as a globalcoordinate system, a local coordinate system relative to the patient 24,and/or a local coordinate system relative to some portion of thetracking system 28, such as the working tool 40 or the trackable device26. The tracking system 28 includes a computer processor arrangement 34adapted to implement a navigation routine 36 that is capable ofdetecting deformations of the trackable device 26 and compensating forat least some detected deformations during a navigation procedureimplemented with the navigation system 20, as described in furtherdetail hereinafter.

The computer-implemented tracking system 28 may take a variety ofdifferent forms, wherein a computer processor arrangement preferably isadapted to receive data from one or more sensors 38 relative to thelocations of the tracking points 30, track the positions of the trackingpoints 30 relative to at least one coordinate system, and correlate thetracked positions to the positions of one or more additional features,such as a portion of the patient 24, the work tool 40, and/or virtualwork plan information, such as a proposed osteotomy, suture, and thelike. In the exemplary form shown in FIG. 1, the tracking system 28includes the computer processor arrangement 34 and one or more sensors38 adapted to sense the positions of the tracking points 30 remotely.The sensors 38 may include cameras, such as CCD cameras, CMOS cameras,and/or optical image cameras, magnetic sensors, radio frequency sensors,or any other sensor adapted to sufficiently sense the position of thetracking points 30. In the present exemplary arrangement, the sensorsare in the form of cameras, which may be, for example, carried by astand, secured to a wall of an operating room, and/or may be secured tothe working tool 40 in a manner well understood in the art. However,other arrangements of the sensors 38 are also possible. Each of thecameras 38 is adapted to capture a view of the tracking points 30 from adifferent position, and the computer processor arrangement 34 includessoftware and/or hardware having computing instructions adapted tocalculate the location of each tracking point 30 relative to thecoordinate system 32 by triangulation of the different views of the sametracking points. Optionally, one or more cameras may be carried by thesurgical tool 40, and the computer processor arrangement 34 may includecomputing instructions adapted to calculate the pose of a tool 40relative to a pose of the work target 22 based on images of thetrackable device 26 and/or the tracking points 30 captured by the one ormore cameras 38 on the tool 40. The computer processor arrangement 34includes hardware and/or software arrangement to implement the variousroutines and/or methods disclosed herein. In the exemplary arrangementof FIG. 1, the computer processor arrangement also includes inputoutput/devices, such as a keyboard 42 and a display monitor 44, and oneor more data communication devices, such as data communication cables 46and/or wireless communication devices 48. The computer processorarrangement 34 further has access to one or more databases 47 forstoring various data. Optionally, the computer processor arrangementincludes a data connection to a larger computer network 49 such as viathe internet, wide area network, local area network, or other computernetworking means. Additional and/or alternative and/or fewer hardwareand programming components may also be implemented as part of thecomputer processor arrangement 34 as would be understood in the art.

As illustrated in FIG. 2, the navigation routine 36 implements aplurality of method steps that, when suitably implemented together withother components of the navigation system 20, enable the navigationsystem to track the position of the work target 22, and optionallyadditional components such as the tool 40, relative to at least onecoordinate system. The navigation routine 36 may be implemented insoftware programming code accessed and/or implemented by the computerprocessor 34, for example from the database 47 and/or from a remotesource such as via the internet at 49, and/or may include one or moreASIC configurations, in a manner that would be understood in thecomputer programming and control arts.

A block 50 accesses an initial model of the trackable device, such asthe model 51 shown schematically in FIG. 3. The initial model 51 definesan initial shape based on the initial locations 53 of a set of thetracking points 30 from the trackable device 26 after it has beensecured to the skin of the patient. The initial model 51 may be formedseparately from the navigation routine 36 or it may optionally beobtained and/or formed as part of the navigation routine 36. If formedseparately from the navigation routine 36, the initial model 51 may forexample be stored in the database 47 or accessed via a datacommunication connection from a remote source such as at 49.

In one optional arrangement where the initial model 51 is formed as partof the navigation routine 36, a block 52 obtains the initial locations53 of the tracking points 30 after the trackable device 26 has beensecured to the patient 24, and a block 54 creates the initial model 51of the trackable device 26 from the initial locations 53 of the trackingpoints 30. The initial locations 53 may be in the form of coordinatesrelative to the coordinate system 32 or some other arbitrary coordinatesystem, for example, defined relative to the trackable device 26. Insome arrangements, the initial locations 53 are obtained with one ormore of the navigation sensors 38. In some arrangements, the initiallocations 53 are obtained from image data, such as a pre-operative scan.For example, the trackable device 26 and the patient 24 may be scannedtogether after the trackable device 26 is attached to the patient 24.The initial model 51 is created based on the coordinates of the initiallocations 53 of the tracking points 30, and may be the point clouddefined by the coordinates of the initial locations 53. However, othermodeling techniques for creating the initial model 51 may be used. Forexample, a surface mesh may be generated based on the point cloud andapriori knowledge of the trackable device 26. In some arrangements thenavigation routine 36 may create the initial model 51 of the trackabledevice at block 54 from the locations of the set of tracking points 30obtained from a scan data set, such as a preoperative or intraoperativeMRI, x-ray, or other type of scan, without obtaining the initiallocations 53 with the sensors 38 at block 52.

A block 56 registers the pose of the initial model 51 with an initialposition of the work target 22 in an image data set, such as apre-operative scan image of the patient, as shown schematically in FIG.4. The step of registering may be performed in any manner understood inthe art, such as with a point cloud matching technique, a point to pointregistration technique, and/or any other technique sufficient toregister the pose of the initial model 51 as secured to the patient tothe initial position of the work target 22 in image data set of thepatient. In one arrangement, the registration may include identifying aninitial pose of the initial model 51 relative to the coordinate system32, such as a global coordinate system, from the initial locations 53 ofthe set of tracking points 30. Thereafter, the initial pose of theinitial model 51 is registered to the initial pose of the work target 22in a pre- or intra-operative scan image of the patient, such as an MRI,CT, X-ray, ultrasound, optical image, and the like.

After registering, a block 58 senses deformation of the trackable device26 while tracking the locations of the tracking points 30 during anavigation procedure, as depicted schematically in FIG. 5 and discussedin further detail hereinafter. For example, as seen in FIG. 5, thesensed locations 66′ of the tracking points 30 in the upper right cornerof the trackable device 26 have been deformed downwardly substantiallyfrom initial locations 53′ of the same tracking points 30 as representedin the initial model 51 of the tracking device. The sensed locations 66of the remaining tracking points around the upper, lower, and lower leftcorners, however, correspond closely to the initial locations 53 of thecorresponding tracking points 30. This deformation could be caused, forexample, by excessive pressure against the nearby area of the skin ofthe patient near the upper right corner of the trackable device 26.

At a block 60, a refined model 62 of the trackable device 26 thatcompensates for the deformations sensed in block 58 is created, anexample of which is shown schematically in FIG. 6. The refined model 62is a model that based on a modification of the sensed locations of thetracking points after the initial model 51 has been registered to theimage of the work target 22. The step of creating the refined model 62may include excluding some tracking points that are deformed more than aselected deviation threshold from the initial shape of the trackabledevice 26 and/or may include adjusting a sensed location of a deformedtracking point relative to sensed locations of one or more of the othertracking points, as is explained in further detail hereinafter. In theexample of FIG. 6, the refined model 62 is based on a set of the sensedlocations of the tracking points that excludes the locations of trackingpoints that have been deformed beyond the deviation threshold. Thus,continuing with the example started above regarding FIG. 5, the refinedmodel 62 in FIG. 6 excludes the tracking points of the upper rightcorner of the device, as shown schematically in the region 63 of therefined model 62 where no tracking points are shown. In this way, therefined model 62 is based on the sensed locations 66 of the trackingpoints 30 that have not been deformed excessively, while removing theexcessively deformed tracking points from the refined model 62. This canimprove the accuracy of the pose of the refined model 62 relative to theposition of the work target 22 without necessitating interrupting thenavigation procedure to re-register the trackable device 26 or otherwisere-set the entire setup.

Thereafter, a block 64 calculates the current position of the worktarget 22 from the refined model 68 in any suitable manner. Of course,it is understood that, if the sensed locations 66 of the tracking points30 are not deformed beyond the acceptable limit, the navigation routine36 may skip the block 60 and calculate the current position of the worktarget 22 directly from the model defined by the sensed locations 66without performing further refinements to the model.

During a normal surgical navigation procedure, the blocks 50 and 56, andoptionally the blocks 52 and 54, are normally performed a single timeduring a setup of the navigation procedure, and the blocks 58, 60, and64 are normally iteratively repeated during the remaining course or someperiod of the navigation procedure until the navigation procedure isinterrupted or ended for some reason. However, the iteration of theblocks 58, 60 and 64 is optional and may be performed only once and/ormay include additional method steps interspersed therein.

The steps of sensing the deformations of the trackable device 26 andrefining the model, as performed in the blocks 58 and 60 of thenavigation routine 36 may be performed according to various differentspecific methods. In one exemplary method, the step of sensingdeformations at block 58 includes tracking the subsequent locations 66of the tracking points 30 after the initial model 51 has been registeredto the initial location of the target 22 in the scan image. Thesubsequent locations 66 define a deformed model 68 having a deformedshape, which may be simply the shape of the point cloud defined by thesensed locations 66, as shown schematically in FIG. 5. Thereafter, thedeformation of the trackable device 26 is identified based on one ormore differences between the deformed shape 68 and the initial shape 51.

In the exemplary arrangement of FIG. 7, these steps are performed aspart of the block 58. After the trackable device 26 has been registeredto the work target 22 in the scan image, a block 70 obtains thesubsequent location a set of the tracking points 30, for example withthe sensors 38 of the tracking system 28. A block 72 identifies one ormore deformations of the trackable device 26 based on one or moredifferences between the deformed shape 68 and the initial shape 51. Withreference again to FIG. 5, in one method, the block 72 matches theinitial shape 51 to the deformed shape 68. This matching may beperformed by transforming the coordinates of the initial locations 55 ofthe tracking points 30 to the subsequent locations 66 of the trackingpoints 30 as sensed by the sensors 38. For example, the transformationmay include an orthogonal transformation that is a result of a leastsquares match of the point cloud formed by the initial locations 55 withthe point cloud formed by the subsequent locations 66. Alternatively,the coordinates of the subsequent locations could be transformed tomatch the coordinates of the initial locations, or both the initial andsubsequent locations could be transformed to be based on a furthercoordinate system. The initial and deformed shapes 51 and 68 are matchedby any suitable algorithm. One exemplary method includes performing aleast squares analysis to find a best fit between the initial shape 51and the deformed shape 68. However, other methods of matching theinitial and deformed shapes may also be used.

After matching the best fit between the initial shape 51 and thedeformed shape 68, a block 74 calculates a spatial deviation of one ormore of the tracking points 30 of the set. As shown schematically inFIG. 5, the spatial deviation 76 may include the distance and/ordirection between the subsequent location 66 of a particular trackingpoint and the transformed initial location 53 of the same trackingpoint. For example, the same tracking point 30 may have a coordinate 53′corresponding to the transformed initial location of the tracking pointand a coordinate 66′ corresponding to the subsequent location of thesame tracking point. The spatial deviation 76′ in this example is equalto absolute value of the distance between the coordinate 53′ and thecoordinate 66′. In some arrangement, the spatial deviation 76 mayinclude both distance and direction between the coordinate 53′ and thecoordinate 66′. However, other methods of calculating the spatialdeviation 76′ may also be used. This calculation may be performed, suchas iteratively, on all of the tracking points 30 in the set that definethe deformed model 68 and/or can be performed on selected trackingpoints within the set.

A block 78 thereafter determines if one or more of the tracking points30 is deformed (beyond an acceptable amount). In some arrangements, atracking point 30 is considered to be deformed if it has a spatialdeviation 76 that exceeds a selected deviation threshold and/or islarger than the spatial deviation of one or more of the other trackingpoints 30. The deviation threshold may be selected and/or defined as astatic value and/or as a dynamic value. A static value may include anunchanging value, for example, a specific preselected distance abovewhich the spatial deviation is considered to be too large and therebyconsidered an error. A dynamic value may change as a function of one ormore selected parameters, for example, by being based on a comparison ofthe spatial deviation for a particular tracking point 30 in comparisonto the spatial deviations of one or more other tracking points 30 in theset defining the models 52 and/or 68. However, other methods ofselecting the deviation threshold may also be used. This determinationmay be performed individually on a point by point basis, such as bycomparing each spatial deviation 76 with the deviation threshold, and/oror may be based on an agglomeration of a larger set of the trackingpoints, such as by comparing an averaged or composite spatial deviationof two or more of the tracking points 30 to the deviation threshold. Inone exemplary arrangement, the spatial deviation of a particulartracking point 30 is compared with a pre-defined static deviationthreshold. In addition, the difference of the spatial deviations of thetracking points with respect to each other is also considered. Thetracking point 30 that exceeds a static pre-defined deviation threshold,for example about 2 mm, and that has the largest spatial deviationrelative to the other tracking points is removed from the initial model.Then the iterative process starts with the refined model (less onetracking point) being matched with the initial model. Only one point,such as the worst tracking point (i.e., the tracking point with thelargest spatial deviation), is excluded per iterative step. If there isstill a second tracking point that both exceeds the deviation thresholdand has the greatest deviation with respect to the other trackingpoints, then the iterative process repeats, and so on, until no furthertracking points are removed from the model or the model does not haveenough tracking points for suitable navigation. However, if all theremaining tracking points of the deformed model are not above thedeviation threshold, then the target position is calculated. Similarly,if all the tracking points would have the same deviation value/vectorabove the deviation threshold, the model may not be altered because thisdeformation can be assumed to be a uniform movement of the patient 24and/or trackable device 26 (e.g., translation of the entire patient)rather than a deformation of the trackable device 26. If one or more ofthe tracking points 30 has a spatial deviation greater than thedeviation threshold and has the greatest spatial deviation of all thetracking points, then the navigation routine 36 advances to block 60 torefine the model before calculating the target position at block 64. Ofcourse, if the tracking points of the deformed model 68 are notconsidered to be deformed, then the navigation routine 36 may optionallyadvance directly to block 64 to calculate the target position based onthe deformed model itself.

In some arrangements, block 60 includes refining the model by removing,such as by excluding, one or more tracking points 30 from the refinedmodel 62 when the tracking point has a spatial deviation above thedeviation threshold. In this arrangement, the refined model 62 therebyhas fewer tracking points than the initial model of the trackabledevice, as illustrated by way of example in FIG. 6. Thus, the refinedmodel 62 is based on a reduced set of tracking points without thesubsequent location of the removed tracking point. Optionally, thedeformed tracking point is removed from the refined model 62 withoutmodifying the shape of remaining portions of the deformed model 68. Inanother arrangement, the block 60 may refine the model by adjusting thesubsequent location of one or more of the tracking points 30 withoutremoving the tracking point from the set. In this arrangement, when theposition 66′ of a sensed tracking point is considered to be deformed,the location of the deformed tracking point in the deformed model 68 maybe adjusted into a corrected position in the refined model. The sensedlocation of a tracking point may be adjusted relative to the sensedlocations of the other tracking points in the set as a function of therelative positions of the tracking points to each other and the spatialdeviations of these relative positions and/or relative distances. Thisadjustment may be performed based on a finite element method analysis,for example based on a force spring model assumption of movements of theskin of the patient 24. In this method, the spatial deviation 76′ of adeformed tracking point 30 may be equated to a resultant force. A set offorces applied to the spring model may be calculated to shift thedeformed tracking point back toward the original or initial position 53′in the initial model. The shift may result in the shifting of one ormore of the remaining tracking points 30 in the refined model. However,other tracking points 30 in the refined model may be assumed to remainstatic relative to the position of the work target 22. In this way, thecorrected position of the deformed tracking point may be retained in therefined model rather than omitted from the refined model. In addition,the block 60 may apply both of these methods of refining the modeland/or other methods of refining the model so as to compensate fordeformations of the tracking points 30 after the initial positions ofthe tracking points have been registered to the initial position of thework target 22 in the image data set.

After refining the model at block 60, the navigation routine 36 mayoptionally include one or more sufficiency checks such as at blocks 80and 82 as shown in FIG. 7. The optional block 80 determines whether therefined model 62 is still sufficient for use in the navigationprocedure. In one exemplary arrangement, if the step of refining themodel includes removing one or more deformed tracking points from therefined model 62, the block 80 determines if the remaining set oftracking points in the refined model includes at least some predefinedminimum number of tracking points that are considered to be acceptablefor reasons of accuracy or otherwise. If not, then the refined model 62is considered to be insufficient, and a block 84 provides a notificationto a user that the model is no longer sufficient for navigation, such asby ending the navigation routine 36 and/or providing a warning signal.If the refined model 62 includes the minimum number of tracking pointssuch that the model is considered sufficient, then the navigationroutine may continue. In some arrangements, the steps of matching theinitial and deformed shapes, calculating spatial deviations, andrefining the model may be iteratively repeated after the block 80 untileither no further tracking points are removed from the set of trackingpoints forming the refined model 62 or the refined model includes fewerthan the predefined minimum number considered acceptable. Optionally,the block 80 may calculate an averaged spatial deviation of up to all ofthe tracking points in the set of tracking points defining the deformedmodel from the spatial deviations, and the navigation routine is endedat block 84 when the averaged spatial deviation exceeds a selectedvalue.

Optional block 82 determines whether an estimated error in thecalculated location of the work target will be within an acceptableerror range. In one arrangement, this is performed by estimating anexpected error of the calculated current position of the work target 22based on the initial model 51 and the deformation of the trackabledevice 26. Such estimation may be performed according to any desiredmethod and/or based on any desired set of parameters. If the estimatederror in the calculated position of the work target is considered to beunacceptable, for example by exceeding a predefined maximum errorthreshold for the work target, block 86 provides a notification to theuser, such as with a warning message, error message, and/or ending thenavigation routine 36. If, however, the estimated error is considered tobe acceptable, such as by being within the predefined maximum errorthreshold, block 64 then calculates the current position of the worktarget 22 based on the refined model 62.

Turning now to FIGS. 8 to 11, the trackable device 26, unlike trackabledevices in many previous systems, does not need to have a rigid or evenfixed shape, although a rigid or fixed shape trackable device 26 may beused with the tracking system 28 if desired. Rather, due to thecapabilities of the navigation routine 36 to sense and compensate fordeformations of the trackable device 26, it may be formed so as to allowone or more of the various tracking points 30 to shift, such as bydeforming, relative to other ones of the tracking points 30 during anavigation procedure. Thus, any one of the following exemplary trackabledevices 26 a-d may be used as part of the navigation system 20.

In FIG. 8, a trackable device 26 a is secured to the skin of a patient24. The trackable device 26 a includes a flexible substrate 90configured to be secure to the skin of the patient, for example withadhesive or straps. The flexible substrate 90 is in the form of a sheethaving a relatively thin thickness that has unlimited flexibility,although in some arrangement it may have limited flexibility, such as bybeing flexible in only a limited number of directions. The trackingpoints 30 are carried by the flexible substrate 90. The tracking points30 are in the form of optical tracking points, such as LEDs or passiveoptical marks that are visible to sensors 38 in the form of cameras.Although the following description refers to the use of LEDs forexemplary purposes, tracking systems that use passive visual markers mayalso be used with the systems and methods described herein. The trackingpoints 30 are disposed on one side of the substrate 90, and optionallyan adhesive is disposed on the opposite side of the substrate forattaching the substrate 90 to the skin of the patient 24. The flexiblesubstrate 90 is in the shape of a frame surrounding a central window 92through a central portion of the trackable device 26 a. The frame isshaped and sized such that the window 92 is large enough to encompassthe area of the patient above the work target 22 and provide a surgeonwith sufficient room to access the work target through the window 92without excessively disturbing the shape of the frame. The frame isillustrated as being rectangular; however, other shapes are alsopossible. The number of tracking points 30 carried by the flexiblesubstrate 90 is preferably between 20 and 40 optical emitters disposedat regular intervals around the entire circumference of the frame.However, fewer or more tracking points 30 may be disposed on theflexible substrate 90 and may be grouped in different arrangements. Thetrackable device 26 a optionally includes a data communication link 94suitable for sending and/or receiving data to and/or from the trackingsystem 28. The data communication link 94 may include a wirelesscommunication link, for example capable of communicating with thewireless communicator 48 of the computer processor 34. The datacommunication link 94 may include a hard-wired communication link, forexample for connection with the cable 46 to the computer processor 34.Further optionally, the tracking points 30, if optical emitters such asLEDs, may be selectively activated in response to command signalsreceived from the tracking system 28 through the data communication link94. Also optionally, information regarding any physical constraints ofhow the tracking points 30 can move relative to each other may becommunicated to the tracking system 28 with the data communication link94. Such information may include information relative to the flexibilityof the substrate 90, rigidity of the substrate 90, and/or the type ofconnection between tracking points 30 associated with the trackabledevice 26 a. The information may be contained in one or more memorydevices associated with the trackable device 26 a, such as anon-volatile memory also carried with the substrate 90.

FIG. 9 shows another form of a trackable device 26 b suitable for usewith the navigation system 20. Similar to the trackable device 26 a, aplurality of tracking points 30, in the form of optical emitters such asLEDs, are disposed on one side of a flexible substrate 90 which isadapted to be secured to the skin of a patient with for example adhesivedisposed on the opposite side of the flexible substrate. The trackabledevice 26 b also includes a data communication link 94 and optionally amemory as described with regard to the trackable device 26 a. Thedifference, is that the arrangement of the tracking points 30 is notnecessarily in a square or rectangle as in FIG. 3. Rather, while theflexible substrate 90 forms generally the shape of a rectangular orsquare frame surrounding a central window 92, the tracking points 30 arearranged in clusters 95, here generally cross- or star-shaped clustersof four LEDs each, around the circumference of the frame shape of theflexible substrate 90. This may allow the individual clusters 95 oftracking points to be recognized individually as separate uniquelyshaped tracking points 30 collectively. However, remaining portions ofthe trackable device 26 b are substantially functionally the same asdescribed with respect to the trackable device 26 a.

FIG. 10 shows another exemplary trackable device 26 c including a numberof different and separate substrates 90 a, 90 b, etc., for example inthe form of patches. Each substrate 90 a, 90 b is configured to besecured to the outer surface of the patient spaced apart from the othersubstrates if desired. Each substrate 90 a, 90 b carries at least one ofthe tracking points 30. In some arrangements, one or more of thesubstrates 90 a and 90 b may carry multiple tracking points 30, forexample, similar to the clusters 95 shown in the arrangement of FIG. 9.Although any number of separate substrates 90 a, 90 b may be used toform a trackable device 26 c, preferably between 10 and 40 separatesubstrates are provided. In the exemplary form shown in FIG. 10, twentyfour separate substrates 90 a, 90 b, etc. are attached to the skin ofthe patient 24. The trackable device 26 c also includes a datacommunication link 94 and may also include a memory as described withregard to the trackable devices 26 a and 26 b. With the trackable device26 c, the substrates 90 a, 90 b may be selectively secured to the skinof the patient in almost any conceivable arrangement desired, such as ina grid as shown in the example of FIG. 5. In such a configuration, thetrackable device 26 c may use passive visual patches used in combinationwith a camera, such as a webcam, carried by the surgical instrument 40to track the positions. Once secured to the skin of the patient 24, thetrackable device 26 c performs functionally similar or the same as thetrackable devices 26 a or 26 b.

FIG. 11 shows another exemplary trackable device 26 d, in which thesubstrate 90 is shaped in the form of an H or I, and the tracking points30 are spaced apart relatively evenly across the shape of the flexiblesubstrate 90. The remaining portions of the trackable device 26 d arefunctionally similar or identical to the corresponding components of thetrackable devices 26 a through 26 c.

The tracking points 30 are selected to be sensed by the sensors 38 ofthe tracking system 28. Thus, depending on the type of sensor 38 used bythe tracking system 28, the tracking points 30 may include LEDs,reflective surfaces, such as metal spheres, a reflective pattern, amagnetic coil, and/or an optically identifiable geometric shape thatuniquely defines position and/or orientation of the tracking point 30.In some arrangements, at least one of the tracking points 30 may be anoptical target that uniquely defines a unique pose, and the trackingsystem 28 includes a camera such as an optical video camera adapted tocapture the image of the optical target. In this arrangement, thecomputer processor arrangement 34 is adapted to implement a trackingroutine that calculates the pose of the optical target from the capturedoptical image of a single tracking point 30.

Next, an anticipated exemplary method of using the navigation system 20is described when it is adapted for use as a surgical navigation systemas shown schematically in FIG. 1. In a spinal procedure, for exemplarypurposes, the trackable device 26 is secured to the skin of the patient24 in the area of the work target 22, such as a specific targetvertebrae. If the trackable device 26 a is used, for example, theflexible substrate 90 is located to surround the target vertebrae sothat the surgeon can gain access to the target vertebrae through thewindow 92. Thus, a mathematically strong distribution of tracking points30 is provided around the work target 22 to provide a high accuracy, lowerror position calculation during the navigation procedure.

After the trackable device 26 a is secured to the patient 24, it isregistered to a pre-operative image of the patient that includes animage of the target vertebrae. In one arrangement, the initial locations53 of the tracking points 30 are gathered by the tracking system 28 withthe sensors 38. Thereafter, the navigation routine 36 creates theinitial model 51 from the initial locations 53, and registers theinitial model 51 to the pre-operative scan image so that the actualposition of the trackable device 26 a is registered with the position ofthe actual target vertebrae and the image of the target vertebrae in thepre-operative scan image. Alternatively, a pre-operative image of thepatient 24 with the trackable device 26 a already attached may beobtained.

After registration is completed, the surgical procedure advances withthe aid of the navigation system in a manner generally understood in theart. If the trackable device 26 a is distorted during the navigatedportion of the surgical procedure, the navigation system 20 can detect,and in some cases compensate for the distortions, so as to allow thesurgical procedure to proceed without having to reset system, forexample by having to re-register the trackable device 26 a to thepatient or the pre-operative image.

In general, use of a non-rigid trackable device secured directly to theskin of the patient 24, such as the trackable device 26 a, can improveaccuracy of the navigation procedure by reducing the possibility ofbeing bumped or sagging. The present navigation system improves onprevious systems that used non-rigid trackable devices by being able todetect and, in some circumstances, compensate for potential distortionsof the trackable device 26 a during the navigated surgical procedure.

The features described in relation to the exemplary arrangements shownin the drawings can be readily combined to result in differentembodiments, as suggested previously above. It is apparent, therefore,that the present disclosure may be varied in many ways. Such variationsare not to be regarded as a departure from the scope of the invention,and all modifications within the scope of the appended claims areexpressly included therein.

The invention claimed is:
 1. A navigation system for tracking theposition of a work target located inside a body, wherein the body iscompressible and has a distortable outer surface, the navigation systemcomprising: a trackable device comprising a flexible substrateconfigured to be secured to the distortable outer surface of the body,and a plurality of tracking points carried by the same flexiblesubstrate, wherein the plurality of tracking points are configured to bemoveable relative to each other when the flexible substrate of thetrackable device is secured to the distortable outer surface of thebody; and a computer-implemented tracking system that is adapted toremotely track the positions of each of the plurality of tracking pointsrelative to a coordinate system, the computer-implemented trackingsystem including a computer processor arrangement adapted to implement anavigation routine that includes the following steps: accessing aninitial model of the trackable device, the initial model having aninitial shape based on initial locations of a set of the trackingpoints; registering the initial model of the trackable device with aninitial position of the work target in an image of the work target;sensing a deformation of the trackable device with thecomputer-implemented tracking system after the step of registering theinitial model of the trackable device; calculating a spatial deviationof at least one tracking point of the set of tracking points, whereinthe spatial deviation is based on a distance between the initiallocation of the at least one tracking point and a subsequent location ofthe same at least one tracking point after the deformation of thetrackable device; excluding the at least one tracking point from the setof tracking points from the initial model based on the calculatedspatial deviation for the at least one tracking point to form a reducedset of tracking points; creating a refined model of the trackable devicethat compensates for the deformation of the trackable device wherein therefined model is based on the reduced set of tracking points; andcalculating a current position of the work target from the refined modelof the trackable device.
 2. The navigation system of claim 1, whereinthe at least one tracking point of the set of tracking points isexcluded from the set of tracking points from the initial model when thecalculated spatial deviation exceeds.
 3. The navigation system of claim1, wherein the step of sensing the deformation of the trackable devicecomprises: tracking subsequent locations of the tracking points of theset of tracking points after the initial model of the trackable deviceis registered, the subsequent locations of the tracking points of theset defining a deformed shape, wherein the subsequent locations includethe subsequent location of the at least one tracking point; andidentifying the deformation of the trackable device based on adifference between the deformed shape of the trackable device and theinitial shape of the trackable device.
 4. The navigation system of claim3, wherein the step of identifying the deformation of the trackabledevice comprises: matching the initial shape of the trackable device tothe deformed shape of the trackable device.
 5. The navigation system ofclaim 4, wherein the at least one tracking point is excluded from theset of tracking points to form the reduced set of tracking points whenthe calculated spatial deviation for the at least one tracking pointexceeds a deviation threshold and has the largest spatial deviation ofall of the tracking points in the set, with the refined model of thetrackable device created based on the reduced set of tracking pointswithout the subsequent location of the at least one excluded trackingpoint.
 6. The navigation system of claim 5, wherein prior to the step ofcalculating the current position of the work target, the steps ofmatching the initial shape of the trackable device to the deformed shapeof the trackable device, calculating the spatial deviation, andexcluding the at least one tracking point are iteratively repeated untilat least one of: no further tracking points are excluded from the set oftracking points; and the set of tracking points or the reduced set oftracking points includes fewer than a pre-defined number of trackingpoints.
 7. The navigation system of claim 1, wherein the navigationroutine further includes the steps of: estimating an expected error ofthe current position of the work target calculated from the initialmodel and the deformation of the trackable device; and providing anindication to a user when the expected error exceeds a pre-definedmaximum error threshold for the current position of the work target,wherein the indication optionally includes at least one of a warningmessage and ending the navigation routine.
 8. The navigation system ofclaim 1, wherein the navigation routine includes the steps of: sensingthe initial locations of the tracking points of the set of trackingpoints with a navigation sensor configured to measure a position of theset of tracking points relative to the navigation sensor; and creatingthe initial model of the trackable device from the sensed initiallocations of the set of tracking points.
 9. The navigation system ofclaim 1, wherein the trackable device comprises: a flexible substratecomprises a closed or semi-closed frame defining a window in a centralportion of the trackable device.
 10. The navigation system of claim 9,wherein the frame is in the shape of one of a rectangle, a square, acircle, a semi-circle, an oval, a U, and an H defining the window in thecentral portion of the trackable device.
 11. The navigation system ofclaim 1, wherein the trackable device comprises: a plurality of separatesubstrates, each of the plurality of separate substrates configured tobe secured to the distortable outer surface of the body in a locationspaced apart from the other substrates of the plurality of separatesubstrates, wherein each substrate of the plurality of separatesubstances carries at least two of the plurality of tracking points. 12.The navigation system of claim 1, further comprising: a work pieceadapted to be tracked by the computer-implemented tracking system,wherein the computer-implemented tracking system is adapted to track theposition of the work piece relative to the coordinate system, andwherein the navigation routine further comprises the step of:calculating the position of the work piece relative to the position ofthe work target based on a tracked position of the work piece and thecalculated current position of the work target.
 13. The navigationsystem of claim 1, wherein the navigation system is a surgicalnavigation system adapted for use in a surgical operating room, and thetrackable device is adapted to be attached to the skin of a surgicalpatient and to extend around a surgical area on the surgical patientwithout covering the surgical area.
 14. A method of tracking theposition of a work target located inside a body with a navigationsystem, wherein the body is compressible and has a distortable outersurface, the navigation system comprising a trackable device and acomputer-implemented tracking system, the trackable device comprising aplurality of tracking points configured to be secured to the distortableouter surface of the body, wherein the plurality of tracking points areconfigured to be moveable relative to each other when the trackabledevice is secured to the distortable outer surface of the body, and thecomputer-implemented tracking system is configured to remotely track thepositions of the plurality of tracking points relative to a coordinatesystem, the method comprising the steps: accessing an initial model ofthe trackable device, the initial model having an initial shape based oninitial locations of a set of the tracking points; registering theinitial model of the trackable device with an initial position of thework target in an image of the work target; sensing a deformation of thetrackable device with the computer-implemented tracking system afterregistering the initial model of the trackable device; calculating aspatial deviation of at least one tracking point of the set of trackingpoints, wherein the spatial deviation is based on a distance between theinitial location of the at least one tracking point and a subsequentlocation of the same at least one tracking point after the deformationof the trackable device; excluding the at least one tracking point fromthe set of tracking points from the initial model based on thecalculated spatial deviation for the at least one tracking point to forma reduced set of tracking points; creating a refined model of thetrackable device that compensates for the deformation of the trackabledevice wherein the refined model is based on the reduced set of trackingpoints; and calculating the current position of the work target from therefined model of the trackable device.
 15. The method of claim 14,wherein the step of creating a refined model includes excluding the atleast one tracking point from the set of tracking points from theinitial model when the calculated spatial deviation exceeds a deviationthreshold.
 16. The method of claim 14, wherein the step of sensing thedeformation comprises: tracking subsequent locations of the trackingpoints of the set of tracking points after the initial model of thetrackable device is registered, the subsequent locations of the trackingpoints of the set defining a deformed shape, wherein the subsequentlocations comprise the subsequent location of the at least one trackingpoint; and identifying the deformation of the trackable device based ona difference between the deformed shape of the trackable device and theinitial shape of the trackable device.
 17. The method of claim 16,wherein the step of identifying the deformation comprises: matching theinitial shape of the trackable device to the deformed shape of thetrackable device.
 18. The method of claim 17, wherein the step ofcreating the refined model of the trackable device comprises: excludingthe at least one tracking point from the set of tracking points to formthe reduced set of tracking points when the spatial deviation for the atleast one tracking point exceeds a deviation threshold and has thelargest spatial deviation of all the tracking points in the set oftracking points, with the refined model of the trackable device createdbased on the reduced set of tracking points without the subsequentlocation of the at least one excluded tracking point.
 19. A navigationsystem for tracking the position of a work target located inside a body,wherein the body is compressible and has a distortable outer surface,the navigation system comprising: a trackable device comprising aplurality of tracking points configured to be secured to the distortableouter surface of the body, wherein the plurality of tracking points areconfigured to be moveable relative to each other when the trackabledevice is secured to the distortable outer surface of the body; and acomputer-implemented tracking system that is adapted to remotely trackthe positions of a work piece and each of the plurality of trackingpoints relative to a coordinate system, the computer-implementedtracking system including a camera configured to optically sense thetracking points and measure a position of the set of tracking pointsrelative to the camera, and a computer processor arrangement adapted toimplement a navigation routine that includes the following steps:accessing an initial model of the trackable device, the initial modelhaving an initial shape based on initial locations of a set of thetracking points; registering the initial model of the trackable devicewith an initial position of the work target in an image of the worktarget; sensing a deformation of the trackable device with thecomputer-implemented tracking system after the step of registering theinitial model of the trackable device; calculating a spatial deviationof at least one tracking point of the set of tracking points, whereinthe spatial deviation is based on a distance between the initiallocation of the at least one tracking point and a subsequent location ofthe same at least one tracking point after the deformation of thetrackable device; excluding the at least one tracking point from the setof tracking points from the initial model based on the calculatedspatial deviation for the at least one tracking point to form a reducedset of tracking points; creating a refined model of the trackable devicethat compensates for the deformation of the trackable device wherein therefined model is based on the reduced set of tracking points;calculating a current position of the work target from the refined modelof the trackable device; and calculating the position of the work piecerelative to the position of the work target based on a tracked positionof the work piece and the calculated current position of the worktarget.
 20. The navigation system of claim 19, wherein the plurality oftracking points are optical emitters adapted to be activated to emitsignals to be optically sensed by the camera.
 21. The navigation systemof claim 20, further comprising: a data communication link in electriccommunication with the computer-implemented tracking system with thedata communication link adapted to selectively activate the opticalemitters in response to commands received from the computer-implementedtracking system.
 22. The navigation system of claim 8, wherein the stepof creating the refined model comprises: adjusting the sensed initiallocation of the at least one tracking point relative to the set oftracking points to the sensed initial locations of the other trackingpoints in the set of tracking points as a function of relative positionsof the tracking points to each other and spatial deviations of theserelative positions.
 23. The navigation system of claim 1, wherein eachof the plurality of tracking points includes at least one of a lightemitting diode, a reflective surface, a reflective pattern, a magneticcoil, and an optically identifiable geometric shape.